This invention relates to multichannel fiber optic communication networks, and is more particularly concerned with optical amplifier designs for such networks that provide a substantial cost saving through the use of identical components in amplifiers having different functions within a network.
Optical amplifiers must satisfy a number of rigorous technical requirements in order to assure reliable and accurate communications within a fiber optic network. As such, they typically represent a significant component of the overall network cost. An individual optical amplifier may cost tens of thousands of dollars, and a given network may require a large number of such amplifiers.
Amplifiers for metropolitan area networks (MANs) or metro amplifiers, for example, have special requirements related to operation over multiple wavelength channels, including input and/or output power monitoring, constant output power (constant average gain per channel), gain flatness over a wide wavelength band, and telemetry monitoring (also referred to as supervisory channel monitoring)--all with the need to support significant passive loss. All of these requirements must be met at a relatively high output power and moderate noise figure in order to meet stringent systems requirements.
Traditionally, optical amplifiers have been designed with a particular focus on their location and function within a network--for example, node input amplifier, node output amplifier, or line amplifier--without regard to the other amplifiers in the network. This individualized design approach is a major contributing factor to the high cost of optical amplifiers.
Doped-fiber optical amplifiers utilize lengths of optical fiber doped with an element (rare earth) which can amplify light, the most commonly used of such elements being erbium. Another element receiving considerable attention is praseodymium. The doped fiber is driven or "pumped" with laser light at an appropriate frequency which excites electrons within the fiber to produce a population inversion of electrons between high and low energy states. Photons of light from a data signal supplied to the doped fiber stimulate the excited electrons to release more photons, thereby amplifying the data signal.
Because doped-fiber amplifiers do not amplify all light wavelengths equally (that is, they do not provide a perfectly flat gain spectrum) it is necessary to employ gain flattening filters to flatten the gain spectrum. These filters are designed to minimize gain ripple at a predetermined average inversion of the doped fiber (referring to the average inversion over the length of the fiber) and at a predetermined internal gain of the amplifier (referring to the minimum gain provided by the fiber among the gains for different wavelengths of the system). The process for designing gain flattening filters is time consuming and expensive, and the filters themselves are expensive as well. Gain flattening filters thus represent a substantial component of the cost of multi-channel fiber optic amplifiers.